1. Field of the Art
The present invention relates in general to a method for coating a metal part with a synthetic resin material, and more particularly to improvements in the art of applying a resin layer to an outer surface of a metallic core member to produce a resin-coated metal part, by positioning the core member heated to an elevated temperature within a powdered mass of a thermally fusible resin.
2. Description of Related Art
Various resin-coated metal parts are known. FIG. 5 shows an example of such resin-coated metal parts in the form of a pair of lobe-type rotors 4 for a rotary fluid machine of a Roots type such as a supercharger used on an engine of an automotive vehicle to increase volumetric efficiency by forcing a greater quantity of air into the cylinders. The supercharger has a housing which consists of a hollow housing body 2, and a pair of end plates (not shown) which close opposite open ends of the hollow housing body 2, and cooperate with the hollow housing body 2 to define an air-tight pump chamber 3. The housing rotatably supports a pair of parallel support shafts 6, 6 which support the corresponding lobe-type rotors 4, 4 accommodated in the pump chamber 3. The two lobe-type rotors 4, 4 are coupled to each other by a pair of timing gears (not shown) fixed to one end of the corresponding support shafts 6, 6, so that the two rotors 4, 4 are rotated in opposite directions at the same angular velocity, whereby air is sucked into the pump chamber 3 through an inlet 8 formed in the housing body 2, and the compressed air is discharged from the pump chamber 3 through an outlet 10 also formed in the housing body 2.
Each lobe-type rotor 4, 4 consists of a metallic core member 11 and a resin layer 12 of a suitable thickness which covers an outer peripheral surface and opposite end faces of the core member 11. The resin layer 12 is applied to minimize gaps between the two rotors 4, 4, and between the rotors 4, 4 and the inner surface of the housing body 2, and to thereby improve the volumetric efficiency of the supercharger. The core member 11 consists of a pair of lobes, and has a transverse cross sectional shape similar to the shape of a cocoon or peanut shell.
For applying such a synthetic resin coating (hereinafter called "resin layer") to the outer surface of a metallic core member, the present applicants have attempted to practice a method wherein the metallic core member is heated to a temperature higher than a melting point of a thermally fusible synthetic resin while the core member is positioned within a powdered mass of the synthetic resin, so that a portion of the powdered mass surrounding the outer surface of the core member is melted and deposited on the outer surace of the core member. To this end, the core member is immersed into the powdered mass of the synthetic resin accommodated in a container. Alternatively, the core member is first placed within the container and the powdered mass of the synthetic resin is introduced into the container, so as to embed the core member in the powdered mass. Subsequently, the metallic core member is induction-heated to a temperature higher than the melting point of the synthetic resin, by energizing a heating coil which is disposed around or within the container.
The above coating method permits formation of a resin layer on the outer surface of the metallic core member in an efficient manner with relatively simple and less costly equipment. The formed resin layer has a degree of adhesion to the metallic core member which is sufficient in actual practice.
However, the applicants found that the above method of forming the metallic core member by heating the core member while it is embedded in the powdered mass is deficient in several respects. More specifically, heat is likely to be transferred from the heated workpiece to the powdered mass, and so the heating of the workpiece requires a relatively long time, which means a relatively long cycle time or relatively low coating efficiency.
Further, the heating of the workpiece while it is embedded within the powdered mass may easily cause voids or pores within a resin layer to be formed on the workpiece. Once air gaps are formed between the surface of the workpiece and the powdered mass, such air gaps seem to prevent the molten resin material from adhering to to the portions of the workpiece surface adjacent to the air gaps. Accordingly, these air gaps tend to be left within the formed resin layer.
Also, it appears that pores are left within the formed resin layer because of substantially simultaneous melting of a relatively large portion of the powdered resin mass adjacent to the workpiece surface. That is, a relatively large molten portion of the powdered mass is simultaneously deposited onto the workpiece surface. This may possibly cause minute spaces between the resin particles to be trapped in the resin layer to be formed on the workpiece surface. It is generally understood that the existence of pores or voids within the formed resin layer is not desirable. This is particularly so if a large number of pores are present at the interface between the workpiece surface and the inner surface of the resin layer. Such pores at the interface reduce adhesion of the resin layer to the workpiece, and consequently lead to flake-off or separation of the resin layer from the workpiece, i.e., from the metal part during its service.
The deficiencies stated above are encountered not only on a metallic core of a lobe-type rotor of a supercharger, but also on other metal parts which are coated with a synthetic resin.